1. Field of the Invention
The present invention relates to fiber optic sensors and particularly to a depolarized light source for providing a constant optical polarization state of depolarized light to a remotely-located fiber optic sensor.
2. Description of the Prior Art
Due to birefringence in optical fibers there is always a problem in delivering light through a long fiber run to a remote location with a definite polarization state. Input polarization states tend to be altered to different states at the output of the fiber. In general, three different approaches have been employed to solve or circumvent this problem. In a first approach, polarization can be maintained by using a high-birefringence polarization-preserving fiber. In a second approach, polarization can be actively controlled with a birefringence device which produces the desired state at the output in conjunction with a feedback loop. In a third approach, depolarized light can be used which will remain depolarized, independent of fiber birefringence.
In many fiber sensor and coherent communications applications, polarization control at the output end of a fiber is important because two optical signals are being interfered on a detector If the two beams do not have the same polarization, "fading" results. In systems with a remote integrated optic device, generally a fixed linear polarization state of constant amplitude is required at the input of the remote device. Since depolarized light can always be described as the superposition of any two orthogonal, uncorrelated polarization states, the use of depolarized light in such systems will always provide the required polarization state with a penalty of 3 dB optical power loss. The difficulty is that, except for broadband sources which can easily be depolarized using lengths of high birefringence fiber, depolarized light sources do not generally exist.